An interdigital surface acoustic wave transducer has the property of converting electrical to acoustic energy, or vice versa, where and only where two arrays of metallic fingers are interleaved with each other. Devices comprising two or more such transducers printed on the surface of a piezoelectric substrate are manufacturable by photographic techniques common in the integrated circuit industry, and have been widely used as intermediate frequency bandpass filters for television receivers. In the T.V. environment it is quite important that these filters exhibit low acoustic reflectivity. Otherwise "triple transit" echoes can produce objectionable ghost images on the picture screen.
In the past such reflections have been minimized by various stratagems. The interdigitated fingers of at least one of the filter transducers have been of the "split-connected" type, in which each finger is divided into two parts at the same electrical potential. Such fingers are effective to minimize reflections when the impedance of the transducer is high in comparison to the external load or source impedance to which it is connected in the I.F. section of the T.V. set. When crystalline lithium niobate (LiNbo.sub.3) is used as the piezoelectric substrate material, the impedance of the transducers is typically about 300 .OMEGA., while the external load and source circuits encountered in the I.F. circuit are of the order of only 50 .OMEGA.. Thus the conditions for low reflectivity are satisfied with LiNbO.sub.3.
This low reflectivity permits some important design trade-offs to be made, without producing objectionable reflection levels. It is desirable that at least one of the interdigital transducers of the filter have 50% duty cycle single fingers (i.e., fingers which are not split in two, and which have a thickness equal to the spaces between fingers), because this 50% configuration increases the filter production yield (it is less sensitive to defects in the photofabrication process) and also because it has a lower third harmonic response (such filters are quite selective for the intermediate frequency, but do exhibit spurious responses at the third harmonic thereof). A disadvantage of the 50% configuration is that it has a higher reflection coefficient. This can usually be tolerated, however, on a LiNbO.sub.3 substrate because in that case one of the transducers can be of the split-connected type, which has a very low reflection coefficient because of the favorable impedance relationships.
Crystalline LiNbO.sub.3 is quite expensive. It would be desireable to replace that material with a cheaper piezo-electric ceramic, such as lead titanate (PbTiO.sub.3). But ceramic materials are made by a sintering process, and do not have as perfect a surface as a polished crystalline material. Microscopic pits in the surface of such a substrate may cause voids in the metal fingers, thereby reducing the production yield of filters. Moreover the surface acoustic velocity of PbTiO.sub.3 is about 30% lower than that of LiNbO.sub.3. A lower acoustic velocity means a proportionately smaller acoustic wavelength, which in turn dictates proportionately smaller finger dimensions. This makes the production yield even more vulnerable to surface imperfections, because a given size void represents a larger fraction of the finger area.
The production yield of PbTiO.sub.3 -based filters can be improved by using single fingers (instead of the split configuration) for all the transducers, because a given size void then represents a smaller fraction of the finger area. But solid fingers have a large reflection coefficient.
PbTiO.sub.3 also has a higher dielectric constant than LiNbO.sub.3, and therefore the transducer impedances are much lower (of the order of 30 .OMEGA.). This is lower than the 50 .OMEGA. source and load impedances typically encountered in the T.V. environment. When such an unfavorable impedance relationship is encountered, no improvement in reflectivity can be attained by employing split-connected fingers, even if one were willing to tolerate the yield reduction resulting from the split finger configuration.
Thus the use of single finger transducers is indicated on a PbTiO.sub.3 ceramic substrate. Accordingly, it is desireable to find some way of decreasing the reflection levels encountered in such transducers. One way to reduce reflections is to eliminate unnecessary transducer fingers or portions thereof, and thus reduce the number and/or length of reflecting metal edges.
It is common in the art to use "apodization" in at least one of a filter's transducers in order to optimize its frequency response. Apodization involves limiting the active portion of the transducer to an area smaller than the entire transducer area. (The "active" area is that region in which electrically opposed fingers are interdigitated to transduce electrical to acoustic energy or vice versa). The "dummy" or inactive area of the transducer may have many noninterdigitated finger portions, the edges of which all give rise to reflections. If these inactive finger portions could be eliminated, then the total reflecting edge length would be reduced and overall reflectivity lowered.
But if these finger portions are removed, then the surface acoustic velocity in the bare substrate area is higher than that of the finger-filled active area. This causes the transducer to behave like a lens, bending the acoustic wavefronts which pass through it. Such bending not only disperses the acoustic energy, but it also makes the wavefront appear to lose phase coherency as "seen" by the straight fingers of the transducers. Phase coherency in relation to these fingers is essential to the satisfactory operation of the transducers, and in particular to the frequency selectivity of the filter.
Another alternative is to cover the entire dummy area with a 100% blanket of conductive material, which has only one reflecting edge instead of many. But this makes the surface acoustic velocity in the dummy area lower than that of the active area, because the latter is not entirely covered by metal. Therefore the dummy area would again behave like a lens, which cannot be tolerated.
The present invention reduces the reflection level of an apodized transducer by eliminating multiple metal edges in the dummy area; but it does so without distorting acoustic wavefronts from their required flat configuration. Instead of employing either a totally bare or a 100% metallized substrate in the dummy area, this invention employs partial metallization in a unique pattern such that, despite differences in surface acoustic velocities between metallized, partially metallized, and fully metallized areas, the total acoustic transit time is equalized across substantially the entire transducer aperture. This causes all acoustic wavefronts to remain flat as they pass through the transducer. This design is particularly advantageous when PbTiO.sub.3 ceramic or any similar material is used as a substrate.
The features of the invention will be more fully understood by reference to the detailed description of the preferred embodiments which follows, when read in conjunction with the accompanying drawings.